In this application the investigator proposes to analyze Ly-49A and its ligand. In particular, in order to evaluate the potential for an expressed repertoire of these receptors, the allelic forms of Ly-49A will be identified and their specificity determined. This will also permit analysis of the domains of Ly-49A that are involved in binding to MHC class I. The binding site of MHC class I that interacts with Ly-49A also will be determined to ascertain whether the Asn-linked glycosylation site on H-2Dd interacts with Ly-49A, a Ca++-dependent ligand. The applicant will determine if Ly-49A mediates its effects by acting as an inhibitory receptor and the function of Ly-49A in transgenic mice that also express its MHC ligand.